Poor Idle Control Quality-

[Joseph S.]

Need some help! I am in the process of tuning an original 302 with a 30-30 cam (yes a 302 but should be about the same as the 64-65 327/365 with the 30-30 Duntov cam)

Engine was built .030' over with otherwise original components.

My problem is, with idle speed set at 950 and initial timing set (12-15 ATC) when engine speed is accelerated engine it will not reduce back to idle unless I turn down idle speed- then of course the engine stalls.
Engine speed is being held up by centrifugal advance, vacuum advance is ported to a B28 can (all in 8 distributor degrees at 8" vacuum) so not in the picture.
Engine only has 4" vacuum at 950 rpm so I will attempt to loosen the valve adjustment a bit to get vacuum up ( I originally set to .027" cold -I'll open them to about .029").

To reduce the engine speed 'run on centrifugal advance' I changed distributor advance springs using a set from low HP 402 BB distributor. (throttle response was a little better with the original 'looser' springs) That helped the problem but does raise full centrifugal advance engine speed. Thus my high initial timing and radical vacuum advance can.

Also checked for vacuum leaks and cannot come up with any.

Before I start tuning WOT total advance and total advance like to know if I can get some input.

I have read threads with guys with similar issues who were running big aftermarket cams but I haven't read any definitive solutions. Tried factory initial timing setting (4 degrees) and using manifold vacuum on the advance can but problem only got worst .

Thx Joe

[Duke W.]

Re: Poor Idle Control Quality-

Before you mess with the valve lash, convert the vacuum advance from ported to full time.

As you say the optimum spark advance map for a 302 with a 30-30 cam should be about the same as a 327/365 and the OE centrifugal (start @ 700, 24 @ 2350) with a 8" B28 VAC is spot on. The only issue is to set total WOT advance as high in the 36-40 range as the engine will tolerate without detonation.

I've fine tuned a couple of 365s in the last few years. Both had the OE spark advance map, so I just set the dwell at 30 and set total WOT advance at 38-39, then went through the idle speed/mixture adjustment procedure. One had one idle screw out one turn and the other 1.5. Both out 1.5 was best and this pretty much eliminated a noticeable start off stumble.

I set the idle at 900 and total idle advance was in the low 30s, which included 2-3 degrees of centrifugal since it starts at 700. Vacuum was 14", and both started out from a dead stop easily with minimal clutch slippage and pulled without protest from 1000 revs in top gear.

Duke

[John P.]

Re: Poor Idle Control Quality-

Hello Joseph, According to your post, you set initial timing at 12-15 ATC ( After Top Center ). Initial timing should be set BTC ( Before Top Center). Hopefully this is your problem. John

[Joseph S.]

Re: Poor Idle Control Quality-

Thanks John- I actually typed it wrong. I set initial timing at 12-15 degrees advanced or BTDC. Good catch
thx Joe

[Duke W.]

Re: Poor Idle Control Quality-

"Vacuum was 14",

Oops... should be 10" @ 900. The edit function is no longer on the post.

Duke

[Mark E.]

Re: Poor Idle Control Quality-

"Engine speed is being held up by centrifugal advance..." I'm not clear what this means.

When does centrifugal advance begin? Idle can be erratic if centrifugal advance begins at or below idle speed.

[Duke W.]

Re: Poor Idle Control Quality-

As I said in post #2 the 327/365 centrifugal starts at 700 but the 30-30 cam idle quality is very poor below about 900, and the 2-3 degrees centrifugal added at about 900 DOES NOT cause erratic idle quality. The 327/340 in my SWC has had the 327/365 spark advance map since circa 1965, and it idles quite stably. Prior to modifying the spark advance map the idle was unstable and the engine often stalled unless I blipped the throttle every few seconds because the OE 15" VAC did not meet the Two-Inch Rule with the Duntov cam that pulled about 12" at 800-850.

In the case of this 302 the OP has not told us anything about the centrifugal, only that the 8" B28 VAC is tied to a ported vacuum advance source, which needs to be moved to a full time source.

Duke

[Joseph S.]

[Duke W.]

Re: Poor Idle Control Quality-

It could be worn carburetor throttle shaft bores that causes variation in area when the throttle is closed, so you have to blip it to get the idle down to normal. A quick fix is a stiffer throttle spring, but that's temporary. I know this can be an issue with old AFBs... not sure about Holleys.

One test you can run is tie up the centrifugal with a rubber band. Test the B28 VAC with a vacuum pump to make sure it meets start/max vacuum specs and hook it up to a full time vacuum source. Then open the throttle slightly to about 2000 and close gently. Check timing... it should be constant, about 30 degrees (full vacuum advance plus initial), and if the engine won't return to normal idle speed, it must be a carb issue. Run this test several times and look for a consistent result.

Make sure the centrifugal advance mechanism is squeaky clean. I've seen some packed with wheel bearing grease. This will definitely cause the mechanism to stick. Only wipe down the parts with WD-40 for some corrosion protection. Absolutely NO grease or oil of any kind.

Duke

[Mark E.]

Re: Poor Idle Control Quality-

As I said in post #2 the 327/365 centrifugal starts at 700 but the 30-30 cam idle quality is very poor below about 900, and the 2-3 degrees centrifugal added at about 900 DOES NOT cause erratic idle quality. The 327/340 in my SWC has had the 327/365 spark advance map since circa 1965, and it idles quite stably. Prior to modifying the spark advance map the idle was unstable and the engine often stalled unless I blipped the throttle every few seconds because the OE 15" VAC did not meet the Two-Inch Rule with the Duntov cam that pulled about 12" at 800-850.

In the case of this 302 the OP has not told us anything about the centrifugal, only that the 8" B28 VAC is tied to a ported vacuum advance source, which needs to be moved to a full time source.

Duke

I'm just sharing what I've experienced. If centrifugal advance begins below idle speed, idle speed adjustment becomes very finicky. Increasing idle speed causes the timing to advance which causes the idle speed to increase more than expected. The opposite happens when lowering idle speed. Specs for many GM engines (including all I've worked on) begin the advance curve above idle speed- perhaps for this reason.
Maybe 30-30 cam engines are different in this regard.

I don't see how changing from ported to full vacuum for VAC will help with this problem. But the suggestion to check for worn throttle shafts is a good idea.

[Duke W.]

Re: Poor Idle Control Quality-

It's generally best for the centrifugal to start just above idle speed, but in my experience with aggressive centrifugal advance curves on high overlap cam engines a few degrees centrifugal at idle speed is not an issue. The important issue is to have 30-34 degrees total idle advance for best idle quality at the lowest possible engine speed.

Ported vacuum advance can be an issue, particularly with a B28 VAC that starts to pull at 4". A ported vacuum advance signal line may have enough vacuum to pull a varying amount of vacuum advance at idle speed and cause it to hang up or be unstable.

All the OP has to do is set up his spark advance map same as the OE 327/365 engines, which includes full time vacuum advance for the B28 VAC with 36-40 total WOT advance and it should be fine... just as it is fine on a 327/365, but the shorter stroke 302 might like a bit more idle speed like 950-1000.

Duke

[Joseph S.]

Re: Poor Idle Control Quality-

Have been delayed at giving an update account a fuel leak in the primary bowl of the Holley float adjustment which had to have a bushing installed to fix the leak.
After valve re-adjustment (to .029-.030") vacuum is up to between 8-9" at idle (950 rpm). As Duke commented engine idles best and accelerates off idle best with 30+ degrees total idle advance. I have VAC (B28) set at 12 degrees advance and the rest is a combination of initial timing and 4-6 degrees of centrifugal advance. Need to set WOT and total timing- will get to that after some other outstanding work.
I do have a question: What is an 'OP"?

Thanks to all for your help - more to come- Joe

[Timothy B.]

Re: Poor Idle Control Quality-

What Holley carburetor is on the car? Do you know for sure that the camshaft is a original or a aftermarket? Make sure the secondary throttle blades are closed to where they just don't stick in there bores.

It's entirely possible the camshaft overlap is allowing unused oxygen from combustion to get sucked back into the combustion chambers causing the slow to return idle, do some research on this. If the idle mixture is lean there is oxygen left over after the burn.

How far are the emulsion screws open and how many turns in on the idle speed screw for idle speed.

OP means original poster.

[Joseph S.]

Re: Poor Idle Control Quality-

Timothy, thanks for the response.
The engine is basically stock except for a .030" overbore. It's equipped with a 4053, 780 CFM, vacuum secondary Holley carb, & stock 30-30 cam. Idle screws are set approx 1.5 turns out, I think this carb has fixed emulsion jets.
Vacuum came up to a respectable 8-9" when I loosened valves to .029-030" effectively cutting overlap. Now my VAC holds in, meets the 2" rule and the 6.0 power valve in the carb should be seated at idle.

Total timing at idle is 30 degrees advanced and the engine is running much better. Still have more tuning- more to come.
Joe